1. Field of the Invention
This invention relates to the field of projectile delivery systems. More specifically, the invention comprises an improved projectile assembly having a gas generator in its aft end. The gas generator injects gas into the projectile's wake, thereby reducing the projectile's base drag.
2. Description of the Related Art
Although the present invention can be applied to many different types of projectiles, it was primarily developed as part of a propulsion system for launching 40 mm grenades (such as the U.S. Army's M433). The invention incorporates many features described in greater detail in prior U.S. Pat. No. 7,004,074 to Van Stratum (2006), which is hereby expressly incorporated by reference.
Metallic cartridges have been used to encapsulate solid propellants for many years. In recent years other materials have been substituted for the traditional brass, but the principles of operation remain the same: A projectile is seated in the open mouth of a cartridge case containing solid propellant. Ignition of the propellant is provided by percussive or electrical means. The burning propellant generates pressurized gas which forces the projectile out of the mouth of the case and then typically down a rifled bore.
The launching of a 40 mm grenade involves the same principles. The main difference, however, is the size and mass of the projectile. A typical shoulder-fired military weapon launches a projectile weighing less than 30 grams at a relatively high velocity (700-1,000 meters per second). In contrast, a 40 mm grenade weapon launches a projectile weighing over 200 grams at a relatively low velocity (70-80 meters per second). Thus, while the operating principles between the two types of weapons are the same, they can be said to operate in different regimes.
Since the human operator can only withstand a fixed amount of recoil, one cannot merely scale up the cartridge of a shoulder-fired rifle and create a useable weapon for launching 40 mm grenades. The design considerations are different. The incorporated U.S. Pat. No. 7,004,074 illustrates and describes an effective approach to the problem of launching large masses at low velocities. The '074 invention uses a high-pressure cartridge embedded within a low-pressure larger cartridge. A burst cup metering system is used to meter propellant gases from the high pressure cartridge into the low pressure cartridge, thereby accelerating the projectile in a smooth and controlled fashion. This approach helps to reduce the peak recoil loads experienced by a user. The high pressure found within the high pressure cartridge also ensures the reliable ignition and combustion of the propellant it contains.
Once the projectile is free of the rifled bore, it flies along a ballistic path until it strikes its target. The velocity of the projectile diminishes continuously due to aerodynamic drag. One of the significant components of the aerodynamic drag is the “base drag,” which refers to the drag induced by the turbulent wake region immediately behind the projectile. The present invention seeks to reduce the base drag, thereby extending the projectile's range using a given amount of powder. As an alternative, the invention also allows a smaller amount of powder to be used while achieving the same range as a prior art projectile. The smaller amount of powder means reduced recoil for the person firing the weapon.